On Wed, Jun 14, 2006 at 04:32:23PM +0200, Sven Geisler wrote:
For example, You run two queries with two clients and each queries needs to read some indices from disk. In this case it more efficient to read from different volumes than to read from one large volume where the disc arms has to jump.
Hmm. Bad example, IMO. In the case of reading indices you're doing random IO and the heads will be jumping all over the place anyway. The limiting factor there will be seeks/s, and you'll possibly get better results with the larger array. (That case is fairly complicated to analyze and depends very much on the data.) Where multiple arrays will be faster is if you have a lot of sequential IO--in fact, a couple of cheap disks can blow away a fairly expensive array for purely sequential operations since each disk can handle >60MB/s of if it doesn't have to seek, whereas multiple sequential streams on the big array will cause each disk in the array to seek. (The array controller will try to hide this with its cache; its cache size & software will determine how successful it is at doing so.)
Mike Stone
